the pioneer FB drivers were only used in the CS series of the speakers. lot of vintage speakers have hard to come by drivers. you can find them on ebay every now and then but sometimes they'll charge you a good price for it cause drivers today aren't made like the fb drivers(from what i seen) especially the the use of cloth for the surrounds on the midranges and woofers. in good condition and weight the CS-XX line of speakers can go for some good change selling online especially your cs-99.
i didn't use any other drivers in my cs-77a cabs but i did use a large vintage technics floor speaker cabs as an experiment with the fb drivers. giving the super tweeter more height gave it better on axis dispersion and bigger soundfield and since the technics cab were ported on the bottom the woofers had better push-pull excursion. so i'm gonna get better materials and experiment with some wool or carpet and replace th original stuffing used. not only does stuffing reduce cabniet resounce but gives the drivers the illusion it's in a bigger box.